1. Field of the Invention
The present invention relates to a crankcase of an internal combustion engine.
2. Description of Background Art
A crankcase which is one of casings used in an internal combustion engine is required to possess case rigidity. Conventionally, there has been known the constitution in which a product height is ensured to increase case rigidity or the constitution in which concave and convex portions are formed on a wall surface of the crankcase in cross section so as to ensure case rigidity with respect to a miniaturized vehicle such as a motorcycle whose height cannot be increased (see JPA-2003-336543, for example).
However, with respect to the constitution in which a protruding portion is formed on an outer surface of a crankcase, and a boss for mounting an engine is mounted by way of the protruding portion, the protruding portion is vibrated resonating with vibrations generated from the inside of the engine thus giving rise to a possibility that resonance sounds are generated.
To cope with the generation of resonance sounds, it may be possible to increase strength of the protruding portion. In this case, however, a thickness of the protruding portion is increased thus giving rise to a drawback that the weight of the crankcase is increased.
The present invention has been made in view of the above-mentioned circumstances, and it is an object of the present invention to provide a crankcase of an internal combustion engine which can prevent the generation of sounds from a protruding portion of the crankcase and can be light-weighted.